Respective setting devices of a parallel-kinematical machines are comprised, for instance, of a piston which is movable axially in a cylinder and which is fastened at one end to a positioning head via a first joint and to a frame structure via a second joint both of said joints having the form of universal joints. See for instance SE 452 279 and its corresponding European Patent specification 0202206. These joints require two degrees of freedom with respect to movement of the positioning head. Because the setting device is normally in the form of a screw-nut mechanism, there is also obtained a third degree of freedom in the form of a rotary movement of a setting device component. It is necessary to eliminate this degree of freedom, in order to achieve axial displacement. Earlier known joints and their setting device mounts have the form of spherical ball joints which have three degrees of freedom, comprising two angular movements and a rotational movement, wherein one degree of freedom (the rotational movement generated by the setting device) is locked either in the joint itself or in the setting device.
Another known joint has the form of a cardan universal joint which although having only two degrees of freedom (two angular movements) is too large and complicated to provide sufficient rigidity.
The drawback with these earlier known joints resides in insufficient rigidity in the joint and in the joint mounts, or in the complication of the joint construction, wherewith both of these factors result in impaired tolerances with respect to movement of the positioning head.
Another drawback with known joints is that they tend to become excessively large when given sufficient rigidity, a factor which has a limiting effect on the mobility of the positioning head.